I assume this is not supposed to happen (?) 0.7rc2 (think I also experienced this a few times on rc1). Just FYI, I've just saw this in the log while running a long-term benchmark, can't tell what triggered this, unfortunately.
ERROR [pool-1-thread-43] 2010-12-11 13:18:18,570 Cassandra.java (line 3044) Internal error processing batch_mutate java.lang.AssertionError at org.apache.cassandra.db.ColumnSerializer.serialize(ColumnSerializer.java:44) at org.apache.cassandra.db.ColumnSerializer.serialize(ColumnSerializer.java:35) at org.apache.cassandra.db.ColumnFamilySerializer.serializeForSSTable(ColumnFamilySerializer.java:87) at org.apache.cassandra.db.ColumnFamilySerializer.serialize(ColumnFamilySerializer.java:73) at org.apache.cassandra.db.RowMutationSerializer.freezeTheMaps(RowMutation.java:362) at org.apache.cassandra.db.RowMutationSerializer.serialize(RowMutation.java:373) at org.apache.cassandra.db.RowMutationSerializer.serialize(RowMutation.java:351) at org.apache.cassandra.db.RowMutation.makeRowMutationMessage(RowMutation.java:218) at org.apache.cassandra.db.RowMutation.makeRowMutationMessage(RowMutation.java:211) at org.apache.cassandra.service.StorageProxy.mutate(StorageProxy.java:136) at org.apache.cassandra.thrift.CassandraServer.doInsert(CassandraServer.java:446) at org.apache.cassandra.thrift.CassandraServer.batch_mutate(CassandraServer.java:419) at org.apache.cassandra.thrift.Cassandra$Processor$batch_mutate.process(Cassandra.java:3036) at org.apache.cassandra.thrift.Cassandra$Processor.process(Cassandra.java:2555) at org.apache.cassandra.thrift.CustomTThreadPoolServer$WorkerProcess.run(CustomTThreadPoolServer.java:167) at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) at java.lang.Thread.run(Thread.java:662)